Urine will work well, if it's concentrated, said emergency Dr. Ryan Stanton. However, if the urine comes from a well-hydrated person, it may be too dilute and be no better than using plain tap water.
In terms of efficiency, the order, he said, is vinegar, alcohol, sea water, and then urine. The chemicals in the first three are better for neutralizing the sting and deactivating the nematocysts. Nematocysts are the stinging cells of a jellyfish that inject venom into your skin, or into something a jellyfish wants to eat.
Once the venom has been neutralized, you have to get rid of the nematocysts. Wash them off with saltwater, or use something with a definite edge, such as a credit card, to remove the nematocysts from your skin.
Watch a video of "The One with the Jellyfish" below, or at least the Jellyfish-specific part, along with a transcript of the "appropriate section."
Monica: Jellyfish sting! Oh, it hurts! It hurts!! It hurts!!
Chandler: Well, can we help?! You want us to take you back to the house?!
Monica: It's like two miles!
Joey: Yeah, and I'm a little tired from digging the hole.
Monica: Oh damn the jellyfish. Damn all the jellyfish!
Chandler: We've got to do something!
Joey: Well, there's really only one thing you can do.
Monica: What?! What is it?!
Joey: You're gonna have to pee on it.
Monica: What?!! Gross!!
Joey: Don't blame me, I saw it on The Discovery Channel.
Chandler: Y'know what, he's right. There's something like uh, ammonia in that, that like kills the pain.
Monica: Well forget it! It doesn't hurt that (tries to take a step) baaad!!!!
... later, the group confesses
Joey: Well, we have to say something! We have to get it out! It's eating me alive!! Monica got stung by a jellyfish.
Monica: (interrupting) All right!! All right. (walks slowly into the living room) I got stung. Stung bad. I couldn't stand. I-I couldn't walk.
Chandler: (following Monica) We were two miles from the house. Scared and alone. We didn't think we could make it. (He goes to put his hands on Monica's shoulders but for some reason can quite complete the action and pulls back.)
Monica: I was in too much pain.
Joey: And I was tired from digging the huge hole!
Chandler: And then Joey remembered something.
Joey: I'd seen this thing on The Discovery Channel...
Ross: Wait a minute! I saw that! On The Discovery Channel, yeah! About jellyfish and how if you... (stops suddenly and turns to look at Monica) Ewwww!! You peed on yourself?!
Phoebe and Rachel: Ewwww!!
Monica: You can't say that!! You-you don't know!! I mean I thought I was gonna pass out from the pain! Anyway I-I tried, but I-I couldn't...bend that way. So... (looks at Joey.)
Phoebe, Ross, and Rachel: (turning to look at Joey) Ewwww!!
Joey: That's right I stepped up! She's my friend and she needed help! And if I had too, I'd pee on anyone of you! Only, uhh, I couldn't. I got the stage fright. I wanted to help, but there was too much pressure. So-so I uh, I turned to Chandler.
Chandler: (wails loudly into his hands) Joey kept screaming at me, “Do it now! Do it!! Do it! Do it now!!” Sometimes late at night I can still hear the screaming.
Joey: (laughs) That's ‘cause sometimes I just do it through my wall to freak you out.
Watch a video of "The One with the Jellyfish" below, or at least the Jellyfish-specific part.
Image Source: Wikimedia Commons